My original donor is currently declared as sorn, looking at the point system to retain original year of registration should I update my Sierra V5 with details of the zetec engine that I plan to use to help add up the points?
Has anyone done this before?

Thanks for the detail, I decided that I will just update the v5 with the new engine details. I called the dvla to explain that I need to update the v5 with details of an engine and gearbox change (as it's an auto donor). They said just to update section 7 with engine number and include a cover letter stating transmission changed to manual.
I can also provide evidence by way of either garage services invoice or independent inspection (the latter may be slightly difficult)
It takes approx 4 weeks.

Once all done I should receive an arp after IVA and I register with personalised plate at application.

I changed an incorrect engine number whilst my donor was worn, it was never done properly when the car was imported, they had just used part of the chassis number. Just filled in V5 and sent it in, no issue